a type of sedimentary rock composed of layered particles that grade upward from coarser to finer sizes and are thought to have originated from ancient turbidity currents in the oceans. They are integral components of sedimentary deep-sea fans adjacent to the base of continental slopes, and they are also found below the major river deltas of the world where they build features called abyssal cones. In addition, abyssal plains are formed by the accumulation of turbidites beyond the limits of deep-sea fans and abyssal cones in locations where there is a very large sediment supply. Gerard V. Middleton